It has been proposed to sort out unnecessary biological cells by sorting them out and radiating a laser beam thereon with an extremely high precision.
Referring to FIGS. 6 and 7 which show a basic principle of such a biological cell sorter, biological cells C are conducted as an extremely fine flow as indicated by the arrow in FIG. 6, and a detection laser beam L1 produced by a detection laser 33 is impinged upon this flow of biological cells C at a point A via a condenser lens 34. The light L2 dispersed by the cells C or the fluorescent light produced from the cells C as a result of laser radiation is received by a light detector 38 via a condenser lens 36, and is then converted into an electric pulse signal "d" (refer to FIG. 7(a)).
The pulse signal d is supplied to a determination circuit 39 so as to be determined if there is any unnecessary cell to be found in the flow of cells, and when any unnecessary cell is detected supplies a destroy command pulse "e" (FIG. 7(b)) to a delay circuit 41. The delay circuit 41 activates a pulse laser 40 for cell destruction into emitting a pulse laser beam L3 towards the cell to be destroyed at a point B after a certain time delay "t" by sending a pulse "h" (refer to FIG. 7(c)) to the pulse laser 40. This time delay "t" corresponds to the time interval required for the cell flow to travel from the point A to the point B. Thus, the pulse laser 40 destroys the cells which are determined to be unnecessary by the detection circuit 39 and lets pass the cells which are found to be necessary by the determination circuit 39.
However, in reality, there is a certain time lag after the pulse "h" is sent to the pulse laser 40 and a pulse laser beam L3 is actually produced from the destruction laser 40, and should this time lag fluctuate the pulse laser 40 may not be able to destroy intended cells. As a matter of fact, when a solid state laser such as continuous YAG laser is used in combination with a so-called Q switch which makes use of an acoustooptical device for converting laser into a pulse beam, the delay time "td" between the arrival of the pulse "h" and the emission of the laser beam L1 tends to change in dependency upon the frequency of pulse laser emission. Normally, the delay time "td" increases with an increase in the frequency of pulse laser emission. In other words, the more recent the previous laser beam emission was, the longer it takes for the pulse laser to emit the next laser beam. This problem is more pronounced when the duration of the pulse laser beam is short in comparison with the delay times t and td, and when the intensity of the pulse laser beam L3 is so low that it must be focused upon an extremely small point.